Increased sensitivity and accumulation of estradiol in the rat oviduct during early pregnancy.

We have previously reported that a single injection of estradiol-17 beta (E2) given on day 3 of pregnancy (P3) is far more effective for accelerating oviductal transport in the rat, than treatment given on day 1 (P1). In order to quantify this change, dose-response curves were established for six different doses of E2 (range 0.031 to 1.00 micrograms per animal) given on P1, P2 or P3. In addition, a possible mechanism was explored by comparing the plasmatic and oviductal levels of E2 between 30 and 180 min following treatment with E2 on P1 or P3. As the interval from ovulation to treatment was increased, the transport of a larger number of embryos was accelerated and a smaller dose was required. The minimal effective dose decreased 30-fold from P1 to P3, the oviducts accumulated 20% to 90% more E2 on P3 than on P1, tissue levels were 6- to 48-fold higher than plasma levels and the latter did not differ between P1 and P3. It is concluded that the oviduct exhibits increased sensitivity and responsiveness to E2 on P3 and this is associated with greater accumulation of the hormone in the organ, not attributable to higher E2 plasma levels.

Increased sensitivity and accumulation os estradiol in the rat oviduct during early pregnancy

We have previously reported that a single injection of estradiol-17 beta (E2) given on day 3 of pregnancy (P3) is far more effective for accelerating oviductal transport in the rat, than treatment given on day 1 (P1). In order to quantify this change, dose-response curves were established for six different doses of E2 (range 0.031 to 1.00 micrograms per animal) given on P1, P2 or P3. In addition, a possible mechanism was explored by comparing the plasmatic and oviductal levelsof E2 between 30 and 180 min following treatment with E2 on P1 or P3. As the interval from ovulation to treatment was increased, the transport of a larger number of embryos was accelerated and a smaller dose was required. The minimal effective dose decreased 30-fold from P1 to P3, the oviducts accumulated 20 percent to 90 percent more E2 on P3 than on P1, tissuelevels were 6- to 48-fold higher than plasma levels and the latter did not differ between P1 and P3. It is concluded that the oviduct exhibits increased sensitivity and responsiveness to E2 on P3 and this isassociated with greater accumulation of the hormone in the organ, not attributable to higher E2 plasma levels

Importance of uterine expulsion of embryos in the interceptive mechanism of postcoital oestradiol in rats.

This study compares the interceptive effectiveness of 1 microgram oestradiol given as a single s.c. injection at 0900 or 1700 hours on Day 1, 2, 3, or 4 of pregnancy in rats. Increasing the interval from ovulation to treatment accelerated the oviducal transport of a larger number of embryos, the majority of which were lost from the genital tract. However, after treatment at 1700 hours on Day 3 the majority of accelerated embryos were retained in the uterus. The number of implanted embryos on Day 14 was equal to the number of eggs remaining in the tract 24 h after treatment. As a consequence, the highest interceptive effectiveness was obtained with treatment given at 1700 hours on Day 2 and at 0900 hours on Day 3 of pregnancy. Accelerated oviducal transport and uterine expulsion of embryos begin to dissociate after Day 2 of pregnancy in the rat. This explains why the most effective treatments to accelerate oviducal transport are not always the most effective to reduce the number of implantations. These data emphasize the importance of retentive and expulsive properties of the uterus for fertility and infertility.